Phase diagrams are the maps of metallurgy. They define which phases are stable in a material system at any given composition and temperature — making them essential tools for alloy design, heat treatment specification, welding metallurgy, and understanding material behaviour in service.
This reference guide explains how to read and apply phase diagrams, with a focus on the systems most important to engineering practice.
What Is a Phase Diagram?
A phase diagram is a graphical representation of the equilibrium phases present in a material system as a function of temperature and composition (and sometimes pressure). The term “phase” refers to a physically distinct region with a uniform structure and composition — such as liquid, ferrite, austenite, or cementite in the iron-carbon system.
Phase diagrams are constructed from experimental data (thermal analysis, metallographic examination) and thermodynamic calculations (CALPHAD method). They represent equilibrium conditions — actual microstructures in engineering components may deviate significantly from equilibrium due to rapid heating or cooling rates.
Key Terminology
| Term | Definition |
|---|---|
| Liquidus | The temperature above which an alloy is entirely liquid. Solidification begins at the liquidus during cooling. |
| Solidus | The temperature below which an alloy is entirely solid. The solidus marks the completion of solidification. |
| Eutectic Point | A specific composition at which a liquid transforms directly into two solid phases simultaneously at a single (minimum) temperature. |
| Eutectoid Point | A solid-state analogue of the eutectic: one solid phase transforms to two solid phases at a specific composition and temperature. |
| Peritectic | A reaction in which a solid and liquid react on cooling to produce a different solid phase. |
| Solvus | The boundary between a single-phase solid solution region and a two-phase region involving a precipitate. Critical for precipitation hardening heat treatments. |
| Tie Line | A horizontal line drawn across a two-phase region at a given temperature, connecting the compositions of the two phases in equilibrium. Used with the Lever Rule. |
| Lever Rule | A mass-balance calculation used to determine the relative proportions (weight fractions) of two phases in a two-phase region from the tie line. |
How to Read a Binary Phase Diagram
- Identify the axes: The x-axis is composition (usually weight % or atomic % of the second component); the y-axis is temperature.
- Identify the phase regions: Single-phase regions are labelled (e.g., L, α, γ, β). Two-phase regions lie between single-phase regions.
- Find the liquidus and solidus: The upper boundary of the solid+liquid (mushy) zone is the liquidus; the lower boundary is the solidus.
- Draw a tie line: For a two-phase region, draw a horizontal line at the temperature of interest. Where it intersects the phase boundaries gives the composition of each phase.
- Apply the Lever Rule: Weight fraction of phase α = (C₀ − Cβ) / (Cα − Cβ), where C₀ is the overall alloy composition and Cα, Cβ are the phase boundary compositions from the tie line.
- Trace a cooling path: Starting from a point in the liquid field, trace vertically downward (constant composition) to follow the sequence of phase transformations on cooling.
The Iron-Carbon (Fe-C) Phase Diagram — The Foundation of Ferrous Metallurgy
The Fe-C equilibrium diagram is the most important phase diagram in engineering metallurgy. It describes the stable phases in the iron-carbon system from pure iron to Fe₃C (cementite, 6.67 wt% C), covering all steels (up to 2.0 wt% C) and cast irons (2.0–6.67 wt% C).
Key Features of the Fe-C Diagram
| Feature | Composition | Temperature | Significance |
|---|---|---|---|
| Eutectic point (ledeburite) | 4.3 wt% C | 1147°C | Lowest melting point in the Fe-C system; basis for cast iron solidification |
| Eutectoid point (pearlite) | 0.77 wt% C | 727°C (A1) | Austenite → ferrite + cementite; basis for all steel heat treatments |
| A1 temperature | All compositions | 727°C | Lower critical temperature — austenite begins to form on heating |
| A3 temperature | 0–0.77 wt% C | 727–912°C | Upper critical temperature for hypoeutectoid steels — full austenitisation |
| Acm temperature | 0.77–2.0 wt% C | 727–1147°C | Upper boundary for hypereutectoid steels |
| α (ferrite) phase | <0.022 wt% C | <727°C | BCC, soft, magnetic, low carbon solubility |
| γ (austenite) phase | up to 2.1 wt% C | 727–1495°C | FCC, non-magnetic, high carbon solubility — parent phase for heat treatment |
| δ (delta ferrite) | <0.09 wt% C | 1394–1538°C | High-temperature BCC iron; relevant to welding solidification |
Steel Classifications from the Fe-C Diagram
- Hypoeutectoid steels (<0.77 wt% C): Microstructure = ferrite + pearlite. The lower the carbon, the more ferrite and less pearlite. Most structural and engineering steels.
- Eutectoid steel (0.77 wt% C): Microstructure = 100% pearlite. Rails, springs.
- Hypereutectoid steels (0.77–2.0 wt% C): Microstructure = pearlite + cementite network at prior austenite grain boundaries. Tool steels, bearing steels.
- Cast irons (>2.0 wt% C): Classified as white, grey, malleable, and ductile (nodular) iron based on carbon form (carbide vs. graphite).
Other Important Engineering Phase Diagrams
| System | Key Feature | Engineering Relevance |
|---|---|---|
| Fe-Cr | Sigma phase formation, 475°C embrittlement region, ferrite/austenite boundaries | Stainless steel design, duplex steel heat treatment limits |
| Fe-Cr-Ni (ternary) | Schaeffler / WRC diagram (pseudo-ternary for welds) | Stainless steel weld microstructure prediction |
| Al-Cu | Solvus — basis for age hardening of 2xxx alloys | Aerospace aluminium alloy heat treatment |
| Al-Mg-Si | Precipitation of Mg₂Si | 6xxx series alloy (6061, 6082) heat treatment |
| Cu-Zn | Multiple phases (α brass, β brass) with composition | Brass alloy selection and processing |
| Ti-Al (binary) | α/β phase boundary; β transus temperature | Titanium alloy heat treatment and microstructure control |
| Ni-Cr (binary) | Solid solution region; γ′ precipitation in Ni-based superalloys | Superalloy design for gas turbine applications |
Phase Diagrams in Heat Treatment Design
Phase diagrams define the temperature ranges for key heat treatment operations:
- Austenitising temperature — must be above A3 (for hypoeutectoid) or between A1 and Acm (for hypereutectoid) to achieve full or partial austenitisation
- Annealing temperature — full anneal above A3; sub-critical anneal below A1
- Normalising temperature — 50–100°C above A3
- Tempering range — entirely below A1 to avoid re-austenitising
- Solution treatment (aluminium alloys) — within the single-phase α region above the solvus, below solidus
- Ageing temperature — within the two-phase region below the solvus
Phase Diagrams in Welding Metallurgy
During welding, a steep thermal gradient produces all temperatures simultaneously across the weld and HAZ cross-section. The Fe-C diagram (and its modifications for alloy steels) explains:
- Why the HAZ develops a gradient of microstructures from fully austenitised (CGHAZ) near the fusion line to barely affected base metal
- The risk of liquation cracking in the partially melted zone (between liquidus and solidus)
- The effect of peak temperature on grain growth, carbide dissolution, and transformation behaviour on cooling
- The basis for preheat and interpass temperature requirements to control cooling rate and martensite formation
Limitations of Equilibrium Phase Diagrams
Phase diagrams represent equilibrium — the state reached after infinitely slow heating or cooling. In practice:
- Rapid cooling (quenching) suppresses equilibrium transformations and produces non-equilibrium phases (martensite, bainite)
- TTT and CCT diagrams are needed to predict actual microstructures under real cooling rates
- Multicomponent alloys require ternary and higher-order diagrams or CALPHAD computational thermodynamics for accurate prediction
- Non-equilibrium segregation during casting means actual compositions vary at the microscale
This page provides an overview of phase diagram fundamentals. Detailed articles on specific diagrams and their engineering applications are available in our article library.
Related resources: Metallurgy Glossary · Elements Library · Microstructure Articles · Tutorials